1. Field of Invention
The present invention relates to electromagnetic wave propagation well logging dipmeters.
2. Description of Prior Art
U.S. Pat. No. 4,019,126, is one of which applicant is inventor, and is assigned to the assignee of the present application. The structure of this patent involved a well logging dipmeter with radio frequency oscillators having coils causing flux lines to circulate through earth formations adjacent a well borehole. Changes in the adjacent earth formations caused current in the oscillator circuit to change and these changes were recorded as data after transmission to surface electronics. Based on the recorded data, dip of different earth formations could then be obtained as a function of borehole depth.
Other U.S. Patents of interest to the present invention were of two types. U.S. Pat. Nos. 2,167,630; 3,187,752; 3,510,757 and 3,609,521 although the general interest, contained the sensing instrumentation entirely within the sonde. U.S. Pat. Nos. 3,388,323 and 3,068,400 included a portion of the circuitry outside the sonde mounted in pads in contact with the earth formations adjacent the borehole. In U.S. Pat. No. 3,068,400, a current electrode and a measuring electrode were mounted in each of three pads. In U.S. Pat. No. 3,388,323, magnetic field sensors mounted in the pads sensed both a primary magnetic field, which was predominantly based on the magnetic susceptibility of the formation, and also a secondary magnetic field generated by eddy currents, which was indicative of formation electrical conductivity. These field readings were separated in phase separation circuits and used to determine formation dip.
Other dipmeter logs in commercial use have utilized as many as four pads. However, the signal to noise ratio, that is, the indication of formation boundary versus other unwanted signals, at times posed problems.